High speed continuous casting system

ABSTRACT

A continuous casting system for molten steel involving (1) a high cooling capacity, low friction mold characterized by a reticulated skirt and liquid sprays for heat removal from the lower portion of the mold, (2) an intermediate pressure chamber over the spray-cooled part of the mold, (3) an underground structure that defines a pressure chamber, (4) a sequence of idler and pinch rolls therewithin defining a path for a continuously advancing billet that solidifies within the underground pressure chamber, and (5) a gaseous medium within the upper and lower pressure chambers at greater than atmospheric pressure. The high cooling capacity, low friction mold and higher than atmospheric pressure of the gaseous medium in the upper and lower chambers increase frozen skin strength and decrease stresses in the frozen skin, by reducing frictional forces and neutralizing the metallostatic pressure of the molten steel within the billet and hence permit casting of billets and slabs at unusually high casting speeds.

nited States Patent [1 1 Koump [451 Feb. 26, 1974 HIGH SPEED CONTINUOUSCASTING [73] Assignee: Urban Reclamation Technologies,

Inc., Waltham, Mass.

221 Filed: May 30,1973

21 Appl. No.: 365,071

Related US. Application Data [63] Continuation of Ser. No. 109,139, Jan.25, 1971,

Primary E.\'aminer--R. Spencer Annear Attorney, Agent, or Firm-Morse,Altman, Oates &

Bello [57] ABSTRACT A continuous casting system for molten steelinvolving l) a high cooling capacity, low friction mold characterized bya reticulated skirt and liquid sprays for heat removal from the lowerportion of the mold, (2) an intermediate pressure chamber over thespray-cooled part of the mold, (3) an underground structure that definesa pressure chamber, (4) a sequence of idler and pinch rolls therewithindefining a path for a continuously advancing billet that solidifieswithin the underground pressure chamber, and (5) a gaseous medium withinthe upper and lower pressure chambers at greater than atmosphericpressure. The high cooling capacity, low friction mold and higher thanatmospheric pressure of the gaseous medium in the upper and lowerchambers increase frozen skin strength and decrease stresses in thefrozen skin, by reducing frictional forces and neutralizing themetallostatic pressure of the molten steel within the billet and hencepermit casting of billets and slabs at unusually high casting speeds.

7 Claims, 8 Drawing Figures PATENTED'FEB26I974 $794,108

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VALENTIN KOUMP ATTORNEYS HIGH SPEED CONTINUOUS CASTING SYSTEM RELATEDAPPLICATION The present application is a continuation of copendingapplication Ser. No. 109,139, filed Jan. 25, 1971, now abandoned.

BACKGROUND AND SUMMARY The present invention relates to the metallurgyof iron ans steel and, more particularly, to a high speed continuouscasting technique that is designed for incorporation into an integratedhigh speed, continuous steel making system and process involvingsupplying iron base solids, removing non-ferrous metals therefrom,delivering the iron base solids to a continuous electric melter forconversion to an iron base melt, successively transferring increments ofthe iron base melt to a series of discrete vessels, successivelyadjusting the chemical components and thermal energies of the iron basemelt increments to produce steel meeting predetermined specifications,continuously casting the steel melt to form a continuous billet, andcontinuously conditioning and working the continuous billet on line toproduce a finished steel shape.

In a continuous steel making system, in which the individual processes(such as melting, refining, casting and rolling) are of inherentlymatched capacity, are coupled and are operated as integrated components,productivity of the system is limited by the-output of a continuouscasting machine. For example, using a conventional continuous castingmachine, a 4 X 4 inch cross-section billet can be cast at approximately150 inches per minute. On the other hand, a 4 X 4 inch cross-sectionbillet typically enters the first stand of a rolling mill atapproximately 300 to 400 inches per minute. Thus if the casting machineand the rolling mill were coupled, the rolling mill, a heavy capitalinvestment, would be operated at less than 50 percent capac ity.Accordingly, there exists a need for a continuous casting technique thatcan operate at speeds substantially higher than those presentlyavailable.

The primary object of the present invention is to provide a continuouscasting system that can cast semifinished steel atspeeds two to threetimes the speed of prior systems. This system is particularly adaptedfor combination with continuous melting, refining, casting and rollingoperations by which a fully integrated steel making system is providedand in which productivity is high and labor costs and capital investmentare low.

More specifically, the continuous casting machine of the presentinvention comprises, as components, a mold with a reticulated skirt thatinitiates billet formation at high speed. A spray system, to remove theheat from the solidifying billet, around the reticulated skirt, an upperpressure chamber surrounding the skirt portion of the mold, anunderground structure that defines a pressure chamber, a sequence ofsupport and pinch rolls therewithin defining a path for a continuouslyadvancing billet that solidifies within the chamber, water sprays withinthe underground pressure chamber to remove heat from the solidifyingbillet and a compressor for establishing elevated pressure within theupper and lower pressure chambers. The continuous casting process of thepresent invention comprises,-as steps, flowing molten steel through thedie region of a mold to initiate a billet at high speed, cooling thebillet intensively in the skirt region of the mold by sprays, advancingthe billet through a curved path defined by a series of support andpinch roll pairs, applying an inert gas to the billet within the upperand lower pressure chambers at greater than atmospheric pressure inorder to compensate for the metallostatic pressure of molten steel,i.e., to prevent bulging of frozen skin between the supports, andbreak-outs, and cooling the billet as it advances through the path inorder to solidify the surface of the billet as a skin about a moltencore and to increase the thickness of the skin until the billet iscompletely solidi- Other objects of the present invention will in partbe obvious and will in part appear hereinafter.

The invention accordingly comprises the devices and processes, togetherwith their components, steps and interrelationships, which areexemplified in the present disclosure, the scope of which will beindicated in the appended claims.

DETAILED DESCRIPTION OF DRAWINGS For a fuller understanding of thenature and objects of the present invention reference is made to thefollowing detailed specification, taken in connection with theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional elevation illustrating a conventionalcontinuous casting machine and process;

FIG. 2 is a cross-sectional elevation illustrating a high speedcontinuous casting machine and process of the present invention;

FIG. 3 is a cross-sectional elevation illustrating a continuous castingsystem and process of the present invention;

FIG. 4A is an enlarged detail view, partly in cross section, of acomponent of the system of FIG. 3;

FIG. 4B is a side view, partly in cross-section, of the component ofFIG. 4A.

FIG. 4C is a cross-sectional view of FIG. 4A, taken along the line4C-4C;

FIG. 4D is an enlarged cross-sectional view of FIG. 2, taken along theline 4D-4D; and

FIG. 5 is a top plan view of a compound system of the type shown in FIG.3, illustrating the relationships between two mirror image systems ofthe type shown in FIG. 5.

DETAILED DESCRIPTION A schematic diagram of a conventional continuouscasting machine is shown in FIG. 1 as follows. A stream of molten steel11 enters a water cooled copper mold 13 and emerges as a partiallyfrozen billet 15 having a molten interior 17 and a solid skin 19 thatthickens as more heat is removed from the billet. The mold is oscillatedin a vertical direction at a frequency of 2-5 cycles per second and anamplitude ranging between 0.1-0.75 inch to minimize sticking of thesteel to the mold. Below the mold, the billet is supported by rolls 21and is cooled by water sprays 23. The maximum speed of the process isdetermined by the strength of solid skin 19. At low casting speed, solidskin 19 thickens rapidly so that it can retain the column of moltensteel at the center of the billet. As the speed of the processincreases, the thickness of solid skin 19 decreases and its averagetemperature increases. Eventually, a speed is reached at which the solidskin is no longer capable of: (l) supporting the metallo-static pressureof the column of molten steel so that skin failure (breakout) oc- 3 curseither between the bottom of the mold and the first roll support orbetween subsequent roll supports; or (2) withstanding the axial force onthe billet due to friction between the copper mold and the solid skin ortension generated in the billet due to bulging of the solid skin betweenroll supports in the roll rack.

The key factors related to mold performance that limits the castingspeed in conventional continuous casting machines are: (1) a decrease onthe heat flux in the mold, with distance from the liquid level, as aresult of formation of the air gap between the mold wall and the surfaceof the billet (due to thermal shrinkage and shrinkage as a result ofsolidification); (2) variation of heat flux along the periphery of themold as a result of variation of the air gap along the periphery of themold, due to minor misalignments between the mold and the roll rack; and(3) frictional forces between the surface of the partially frozen billetand the mold walls, which when excessive, may tear the frozen skin in orbelow the mold.

' A schematic diagram of a high speed continuous casting machineembodying the present invention is shown in FIG. 2 as follows. Detailsare shown in FIGS. 4A, B, C and D. A stream of molten steel 25 enters amold 27, which includes a water cooled upper die 29 and a lower skirt31. This molten steel emerges from upper die 29 at high speed as abillet with a very thin skin. Lower skirt 31 is cage of the same insidedimensions as the mold, defined by a plurality of spaced ribs. Thebillet advances into the cage where it is cooled intensively by spraysof water or other cooling medium impinging on the steel billet directlybetween the ribs. One such other medium is a low melting point metalsuch as tin. The rate of heat removal from the billet in the cage bydirect contact of sprays and hot billets is substantially higher thanthe rate of heat removal in the lower part of a conventional mold. As aresult, the frozen shell of the billet, emerging from the cage, isthicker and at lower temperature (and therefore stronger) than inconventional continuous casting practice.

Furthermore, the minor misalignments between the mold and the roll rackno longer effect the rate of heat removal from the billet since thebillet is cooled directly by the sprays. In consequence, substantiallyimproved peripheral uniformity of the thickness of the frozen skin isachieved by the preceeding method. Below the mold, the billet issupported by rolls 33 and is cooled by water sprays 35. As will beexplained below, the shape of the rapidly advancing billet is maintainedin part by a high pressure atmosphere. This atmosphere is retained byseal rolls 37 to be described more fully below. An intermediate pressurechamber 39 is positioned about skirt 31 to receive gases escaping fromthe main pressure chamber and to provide partial support to the frozenskirt in the space between the ribs.

Since the area of contact between the billet and the skirt of thepresent mold is smaller than the conventional mold, and a portion ofthe'normal force (due to metallostatic pressure) is relieved by ambientpressure, the total frictional force transmitted by the mold to thebillet is substantially reduced. Because the pressure in the undergroundchamber reduces bulging between the rolls, net frictional force in theroll rack is reduced, thereby decreasing stresses in the frozen skin. Inturn, higher casting velocity can be achieved. The higher than normalrate of heat removal in the mold, a more uniform peripheral distributionof heat flux in the mold, lower friction, small spacing between the ribsof the cage and higher than atmospheric ambient pressure of gas aroundthe cage and in the underground pressure chamber, in the present system,permit casting of steel at substantially higher casting velocity than ispossible in conventional casting machines.

A general schematic view of a continuous casting system of the presentinvention, FIG. 3, includes a holding vessel 10 and a tundish vessel 12,which receives the contents of holding vessel 10 continuously for themaintanance of substantially predetermined weight. Vessel 10 is shown ashaving a cover 14, in which are electric heaters to compensate for heatloss. Vessel 10 is mounted for rocking motion under the control of ahydraulic drive (not shown). Holding vessel 10 is supported on a car 20,which rides by flanged wheels 22 on suitable rails. Molten steel 24 ispoured through a spout 26 from holding vessel 10 into a tundish vessel12. Tundish vessel 12 is provided at its floor with a sequence ofnozzels 30, which successively can be moved into operating positionautomatically as operating nozzels of the sequence become eroded.Tundish vessel 12 is supported on a car (not shown), which rides byflanged wheels on suitable rails (not shown). The metallostatic head intundish vessel 12 is maintained by controlling its tilt about a pivot bya hydraulic drive (not shown).

In accordance with the present invention, the molten stream 36 fromtundish vessel 12, after passing through a high speed mold of the typeshown in FIG. 2, is converted into a solid billet within a pressurechamber 38, enclosing all of the operating components of the continuouscasting system. Molten stream 36 advances from the mold into a roll rack42. Roll rack 42 includes driven rolls 43, idler rolls 45 and watersprays 47. The initial thin solid skin is thickened in roll rack 42.From roll rack 42 the billet, the core of which is solidifying, advancesin an arcuate path between the nips of pairs of bending rolls and pinchrolls 44, 46, idler rolls 49 and straightening rolls 48. The pinch rollsare driven. Cooling sprays extend throughout the length of the billet inthe underground chamber.

As shown in FIG. 5, chamber 38 is defined underground by a steelreinforced, concrete casting. As shown in FIG. 5 chamber 38 is one of apair of like chambers, the other of which is designated 54. Each ofchambers 38, 54 is an independent heremetically isolated pressurechamber that selectively is supplied with inert gas from a pump and gassupply 56 through a valve 60. Two chambers are provided so that when thecomponents of one are under repair,the other is operating on line withthe remainder of the continuous steel making system. When chamber 38 isin operation, tundish vessel 12 is positioned above die opening 40 so asto produce the billet shown at 62. When chamber 54 is in operation,tundish vessel 12 is positioned above a die opening 64 so as to producethe billet shown at 66. Once advanced out of its chamber either billetis advanced into a forming station for processing in the remainder ofthe continuous steel system. A pair of manholes 70, 72 are provided forentry into the underground chamber for repairs during down time. The gasin the underground chamber is permitted to escape from the chamber inthe vicinity of mold 40 in order to provide pressurizing gas for theupper pressure chambe). The pressure in the upper chamber is controlledby a valve that regulates escape of steam and pressurizing gas to theatmosphere.

Preferably, the gas within the pressure chamber is inert, being selectedfrom the class consisting of carbon dioxide, nitrogen, carbon monoxideand hydrocarbon gas. Preferably, the gas pressure ranges from 1/2 to 3atmospheres. 1n alternative systems, air may be used instead of inertgas although, in this case, the steel must be such that the resultingincreased rate of oxidation can be tolerated. in one specific example,the are through which a 2 inch billet advances is approximately 20 feetin diameter, the spacing between the ribs of the cage is 0.5-l inch, thegas is combustion products of natural gas, the gas pressure isapproximately 1.2 to 1.3 atmospheres absolute (.2 to .3 above normal) inthe upper pressure chamber and is approximately 2 atmospheres absolute(1 atmosphere above normal) the underground chamber and the speed of thebillet is approximately 60 feet per minute, which is more than twice theconventional speed of a billetin a continuous casting machine.

Since certain changes may be made in the present disclosure withoutdeparting from the scope of the present invention, it is intended thatall matter shown in the accompanying drawings and described in 'theforegoing specification be interpreted in an illustrative and not in alimiting scope.

What is claimed is:

l. A system including an upper mold and a lower structure defining aplurality of pressure chambers communicating for continuous casting ofsteel, said upper mold comprising an upper portion extending along partof the total length of said upper mold and a lower portion in the formof a reticulated skirt extending along the remaining part of the lengthof said upper mold, said remaining part involving a plurality of ribsspaced apart from 0.2 to 1.0 inch, high volume liquid sprays for heatremoval in said remaining part of the length of said upper mold, saidlower part being surrounded by one of said pressure chambers, said lowerstructure encompassing said other of said pressure chambers, thepressure in said one of said pressure chambers being maintained at 0.1to 0.5 atmospheres above atmospheric pressure depending on the level ofmolten steel in said one of said pressure chambers, said structure beingcomposed of concrete and being situated underground, a series of rollsat least in part in said other of said pressure chambers defining a loopthrough which a continuous length of cast steel is advanced, saidpressure in said one of said pressure chambers being established by agas flowing from said other of said pressure chambers, said pressurepartially supporting the surface of said length of cast steel whichconstitutes partially solidified skin, said gas being substantiallyinert with respect to said continuous length.

within said pressure chamber.

1. A system including an upper mold and a lower structure defining aplurality of pressure chambers communicating for continuous casting ofsteel, said upper mold comprising an upper portion extending along partof the total length of said upper mold and a lower portion in the formof a reticulated skirt extending along the remaining part of the lengthof said upper mold, said remaining part involving a plurality of ribsspaced apart from 0.2 to 1.0 inch, high volume liquid sprays for heatremoval in said remaining part of the length of said upper mold, saidlower part being surrounded by one of said pressure chambers, said lowerstructure encompassing said other of said pressure chambers, thepressure in said one of said pressure chambers being maintained at 0.1to 0.5 atmospheres above atmospheric pressure depending on the level ofmolten steel in said one of said pressure chambers, said structure beingcomposed of concrete and being situated underground, a series of rollsat least in part in said other of said pressure chambers defining a loopthrough which a continuous length of cast steel is advanced, saidpressure in said one of said pressure chambers being established by agas flowing from said other of said pressure chambers, said pressurepartially supporting the surface of said length of cast steel whichconstitutes partially solidified skin, said gas being substantiallyinert with respect to said continuous length.
 2. The system of claim 1wherein said liquid is water.
 3. The system of claim 1 wherein saidliquid is molten metal.
 4. The system of claim 1 wherein said gas isselected from the class consisting of nitrogen, carbon dioxide andcombustion products of hydrocarbon fuel.
 5. The system of claim 1wherein said length of cast steel is a billet.
 6. The system of claim 1wherein said length of cast steel is a slab.
 7. The system of claim 1wherein said loop is entirely within said pressure chamber.